For the study, Associate Professor Simon Griffith and Dr Sarah Pryke of Sydney's Macquarie University separated a fertile female Gouldian finch (Erythrura gouldiae), and her social partner, by removing the male from the cage for 30 minutes.

They then introduced a virgin male into the cage, reports ABC Science.

Griffith says the male immediately began making moves on the female who "responded within the first couple of minutes".

The researchers were puzzled by this infidelity given the high price a female Gouldian finch could pay if her indiscretions were discovered, says Griffith.

Male partners play an active role in assisting a female bird to raise her young, contributing up to 50 percent of care involved in incubating the eggs and feeding the offspring.

"When a male suspects his partner is unfaithful he may desert the female and reduce the amount of care he invests in any offspring," says Griffith.

For extra-pair males however, the copulations are a win-win with the male increasing its reproductive success with limited investment, he says.

Griffith says if the introduced male was genetically superior to the social partner, with one copulation he could fertilise up to 80 percent of the eggs.

The female finch appears to have a mechanism through which to sort good and bad sperm, says Griffith.

"Mr Right's sperm is very much better for her than Mr Wrong's. This means that by occasionally cheating, she is maximising her chances of having healthy offspring because one copulation with good male sperm is better than 30 copulations with bad sperm. So even a little cheating can have big benefits," he says.

The researchers say the findings may even have implications for human infertility.